1. Field of the Invention
The present invention relates to an elastic wave resonator such as a surface acoustic wave resonator or a boundary acoustic wave resonator. In more detail, the present invention relates to an elastic wave resonator including an IDT electrode, subjected to apodization weighting so that a plurality of local maximum values of apodization (i.e., crossing width), occur in an elastic wave propagation direction, and a ladder filter including the elastic wave resonator.
2. Description of the Related Art
In the past, various types of elastic wave resonators have been proposed that utilize a surface acoustic wave or a boundary acoustic wave. For example, in Japanese Patent No. 2645674, a configuration is disclosed where, in an IDT electrode, apodization weighting is performed so that apodization becomes large in a surface acoustic wave propagation direction central portion and the apodization decreases toward an end portion in a surface acoustic wave propagation direction. It is supposed that, owing to this configuration, a spurious response due to transverse modes is suppressed. However, since the apodization becomes large in the central portion of the IDT electrode, maximum apodization becomes large. More specifically, there has occurred a problem that electric power consumption is concentrated in the corresponding IDT electrode central portion and power durability is deteriorated. In addition to this, there has also occurred a problem that since the maximum apodization is large, the size of the surface acoustic wave resonator is increased.
Therefore, in WO 2007/108269, an elastic wave resonator is disclosed that is subjected to apodization weighting so that more than one local maximum value exists in an elastic wave propagation direction. FIG. 9 is a schematic plan view illustrating the electrode structure of the elastic wave resonator described in WO 2007/108269.
In an elastic wave resonator 1001, the illustrated electrode structure is formed on a piezoelectric substrate. This electrode structure includes an IDT electrode 1002 and reflectors 1003 and 1004 disposed on both sides in the elastic wave propagation direction of the IDT electrode 1002. The IDT electrode 1002 is apodization-weighted. More specifically, apodization changes in the elastic wave propagation direction. Here, the apodization weighting is performed so that two apodization local maximum values exist in the elastic wave propagation direction. A region where electrodes connected to different electric potentials overlap each other in the elastic wave propagation direction is a region surrounded by envelope curves A and B in FIG. 9. As is clear from FIG. 9, two portions exist where the apodization becomes a local maximum in the elastic wave propagation direction. Therefore, between two portions E and F where the apodization becomes a local maximum, a portion G exists where the apodization becomes a local minimum.
In the elastic wave resonator described in WO 2007/108269, since the IDT electrode is apodization-weighted in such a way as described above, it is possible to suppress an influence of a spurious response due to transverse modes, and it is possible to reduce the maximum apodization of the IDT electrode. As a result, it is possible to improve power durability. In addition, it is also possible to reduce the size of the elastic wave resonator.
However, there was understood to occur a problem that a heat dissipation property is insufficient. More specifically, when high-frequency power is applied to the elastic wave resonator 1001, the amount of heat generation becomes largest in the portions E and F where the apodization becomes a local maximum. However, it is possible to dissipate heat generated in the portions, by integrating one busbar 1002a of the IDT electrode 1002, connected to a ground potential, with the busbars of the reflectors 1003 and 1004. More specifically, since the portions E and F where the apodization becomes a local maximum are relatively close to the reflectors 1003 and 1004, it is possible to release heat to the reflectors 1003 and 1004 sides.
On the other hand, the portion G where the apodization becomes a local minimum value is located away from the reflectors 1003 and 1004. Accordingly, in the portion G where the apodization becomes a local minimum, while the amount of heat generation is smaller than the portions E and F, it is easy for heat to be accumulated. Accordingly, there has occurred a problem that it is difficult to achieve sufficient dissipation of heat of the elastic wave resonator.
In addition, when a heat dissipation property in the IDT electrode 1002 has been insufficient, it has been likely that a resonance characteristic has deviated from an original resonance characteristic owing to a temperature change.